1. Field of the Invention
The present invention relates to an output circuit of a semiconductor device, and, more particularly, to an output circuit of a semiconductor device in which erroneous operation due to the potential variations of the power supply line, or the ground line is suppressed.
2. Description of the Prior Art
Recently, the operating speed of a semiconductor device, for example, a metal-insulator semiconductor (MIS) device or a metal-oxide semiconductor (MOS) memory device, has been increased more and more. In order to increase the operating speed, a conventional approach has been to increase the W/L of the output transistors, W representing the channel width and L representing the channel length of each output MOS transistor. By increasing the W/L, the fall time and the rise time of the output waveform is shortened. More precisely, when the W/L is increased, the mutual conductance gm is also increased so that when the output transistor turns on or off and the output potential changes from a high level (H level) to a low level (L level) or from the L level to the H level, a large current instantaneously flows through the output transistor. As a result of this large current, the time required for charging or discharging the load capacitances included in the wirings connected to the output of the output transistors is short. Therefore, a rapid rise or fall of the output waveform can be obtained. A large current also flows through one of the output transistors when the power supply line voltage is shifted to a higher level or when the ground line voltage is shifted to a lower level. Usually, the power supply line voltage of integrated circuits is 5 volts.+-.0.25 volts and the ground line voltage of integrated circuits is zero volts .+-.0.25 volts. When the power supply line voltage is shifted to 5.25 volts or when the ground line voltage is shifted to -0.25 volts, an even larger current flows through one of the output transistors because a higher voltage is applied to the gate of one of the output transistors.
Due to the large current which flows through one of the output transistors during a transition of its state, the operating speed of the integrated circuit is increased. However, the large current which flows through one of the output transistors creates the following problem. That is, generally, the power supply line or the ground line, to which the output transistors and the internal integrated circuit are connected, has a resistance and an inductance. When an instantaneous large current flows from the power supply line through one of the output transistors to the ground line, the potential of the power supply line or the ground line is instantaneously changed due to the resistance or the inductance. This instantaneous variation in potential causes a noise for the internal integrated circuit. As a result, during transition of the ouput from the output transistors, erroneous operation of the internal integrated circuit of, for example, a sense amplifier in a dynamic memory or an input buffer or a sense amplifier in a static memory may result, or erroneous data may be supplied to the external integrated circuit.